The present invention relates to a protection device for a laser diode and more particularly to a protection device for a laser diode used in the optical head of a videodisk system.
The laser diode used in the optical head of a videodisk system is supplied both by a DC biasing current and by a variable information-carrying current, the purpose of the DC supply being to fix a biasing point about which then occur variations relating to the reading and writing of information on the videodisk.
This laser diode is conventionally associated with an opto-electronic feedback system, whose return branch comprises a feedback photodiode and whose direct branch comprises a feedback amplifier, so as to bring under control the optical power from the laser and this despite the changes in the transfer characteristic of the diode. In fact, the transfer characteristic of the diode may change depending on the temperature or ageing. Furthermore, the transfer characteristic of the diode is such that beyond a certain threshold, this characteristic is almost vertical. Therefore, an error in the control of the supply to the laser diode risks causing immediate destruction of the laser.
Such a situation occurs particularly when the DC biasing current appears with a certain delay with respect to the information-carrying current. In fact, since the laser diode is not correctly biased when the information-carrying current appears, the signal injected into the feedback amplifier by the feedback photodiode is then very weak with respect to the information-carrying signal also injected into the amplifier. Now, it is the difference between these two signals, considerably amplified moreover by the feedback amplifier, which is applied to the laser diode. It can then be readily seen that there is a risk of destruction of the laser in such a configuration.